DENVER (AP) – A man on Colorado’s death row for killing four people at a Chuck E. Cheese restaurant nearly 19 years ago asked a federal court Tuesday to overturn his death sentence.

Attorneys for Nathan Dunlap told a three-judge panel of the 10th U.S. Circuit Court of Appeals that Dunlap is mentally ill and that his trial lawyers failed to adequately represent him.

Paul Koehler, of the state attorney general’s office, told judges that Dunlap’s previous attorneys had experience in death penalty cases.

Dunlap was sentenced to death after he was convicted of killing three teenagers and a mother of two at the Aurora restaurant in 1993.

He’s one of three men on death row in Colorado, which hasn’t executed anyone in 15 years. The death sentences of six other death-row inmates have been thrown out since Dunlap was sentenced.

“Nathan Dunlap is running out of time. This is his last, best chance,” said defense attorney David Lane, who isn’t representing Dunlap. “If he loses here, his odds of being executed skyrocket.”

One of Dunlap’s victims was 19-year-old Sylvia Crowell, who was shot from behind as she helped close the restaurant for the night.

“The hurt is still going on,” her father, Bob Crowell, told The Denver Post before the hearing. “And we are somewhat anxious that somebody is going to throw a monkey wrench in there and he is not going to be executed.”

The 10th Circuit judges could take months to issue a ruling, and the losing side could then ask for a hearing before all the court’s judges. The court’s final decision could be appealed to the U.S. Supreme Court.